Focused ion beams (FIBs) can be focused to a spot smaller than one tenth of a micron. Because of their small spot size, focused ion beam systems are used to create and alter microscopic structures. Focused ion beams can micro-machine material by sputtering or etching, that is, physically knocking atoms or molecules from the target surface. Focused ion beams can also be used to deposit material, using a precursor gas that adheres to the specimen surface and decomposes in the presence of the ion beam to leave a deposit on the surface. FIB systems are widely used in the semiconductor industry to alter prototype integrated circuits by depositing metallic paths to create new connections and by etching metallic paths to eliminate connections. Using a FIB system to alter a circuit allows a circuit designer to test variations of the circuit without undertaking the lengthy process of modifying the photolithography masks and fabricating a new circuit from scratch.
To deposit a conductive path using a FIB system, the system operator directs a jet of precursor gas, typically an organometallic compound such as tungsten hexacarbonyl, to the surface of the specimen while a focused ion beam scans the area upon which the conductor is to be deposited. A metallic layer is deposited only in the area impacted by the beam. Because the ion beam can be focused to a diameter of less than one tenth of a micron, a very fine conductor can be deposited. An ion beam assisted deposition process is described, for example, in U.S. Pat. No. 4,876,112 to Kaito et al. for a “Process for Forming Metallic Patterned Film” and U.S. Pat. No. 5,104,684 to Tao et al. for “Ion Beam Induced Deposition of Metals.”
It is also known to use an ion beam process to deposit an insulating material. An electrically insulating material may be deposited, for example, before depositing a conductive path to prevent the new conductive path from electrically contacting an existing conductor. U.S. Pat. No. 5,827,786 to Puretz for “Charged Particle Deposition of Electrically Insulating Films” describes a procedure for depositing an electrically insulating material.
At times, a circuit designer needs to include in his circuit a conductive element having greater electrical resistance than the deposited metallic conductor described above. Current ion beam deposition methods that deposit tungsten or other metal-based materials by the decomposition of precursor gases are incapable of depositing conductors having high electrical resistance.
The electrical resistance of any particular conductor is determined by its resistivity, which is a property of the material itself, and its dimensions. The resistance is equal to the resistivity divided by the cross sectional area of the conductor multiplied by its length. That is, for conductors composed of materials having the same resistivity, a longer conductor has more resistance than a shorter one and a thinner conductor has greater resistance than a thicker one.
The resistivities of typical ion-beam deposited materials are typically in the range of between one hundred micro-ohm-centimeter and five hundred micro-ohm-centimeter. While these resistivity values are much higher than those of bulk metals, the typical microscopic dimensions of microcircuit connections result in typical resistances of around 100 ohms. Such a low resistance is effectively a short circuit between the points it connects.
Currently, the only way to deposit a conductor having high resistance is to deposit a long, narrow conductor. To make the conductor long, it is often necessary to deposit the conductor in a winding path. Even so, the highest resistance practically obtainable by this method is a few thousand ohms. There is no method for producing structures having resistances in the range of one megohm to one hundred megohms.
The ability to create high resistance structures would have numerous applications in circuit modification, particularly of analog circuits.